Early automatic tachycardia detection systems have relied solely on heart rate for the diagnosis of pathological tachycardia which is often confused with sinus tachycardia. The basis of these systems was that any rate exceeding a predetermined value resulted in a tachycardia termination response. This predetermined value is generally in the vicinity of 150 beats/min (400 ms cycle length). Recent automatic implantable cardioverter/defibrillators may improve discrimination between tachycardias, but still frequently mistake sinus or other supraventricular tachycardias for ventricular tachycardia, resulting in inappropriate cardiac electrical stimulation.
As stated in "Tachycardia Recognition by Implantable Electronic Devices" by Camm, Davies, and Ward, PACE 10:1175, 1987, physiological tachycardia (i.e., sinus tachycardia) may still be confused with pathological tachycardia, especially when abnormal tachycardia arises during sinus tachycardia or sinus tachycardia is present after successful termination of pathological tachycardia. The Article outlines several methods attempting to advance tachycardia recognition, but none can perfectly detect the onset of pathological tachycardia. Such methods include heart rate change algorithm, probability density function, etc.
Future devices may incorporate atrial sensing as well as the well known ventricular sensing. As a result, ventricular tachycardia with AV dissociation or a non-1:1 VA response will be easily distinguished from both atrial tachyarrhythmias associated with second degree AV block and sinus tachycardia. A problem which arises is to accurately differentiate cases of ventricular tachycardia with 1:1 VA conduction from sinus tachycardia which also has a 1:1 A:V relationship. More precise methods of tachycardia detection are needed to handle this problem.